1. Field of the Invention
This invention relates to methods for reducing nitrogen oxides (NOx) emissions from internal combustion engines. More specifically, the invention relates to a catalyst and method for converting NOx in exhaust gases to NH3.
2. Background Art
One of the biggest problems with internal combustion engines today is how to convert environmentally harmful gases and particulates expelled from the engines to safe and environmentally benign products. An internal combustion engine transforms gasoline, diesel, or some other type of fuel into work or motive power through explosive combustion reactions. These reactions also produce harmful byproducts such as carbon monoxide, hydrocarbons (in this application, hydrocarbons refer to short-chain hydrocarbons commonly found in gasoline, diesel or exhaust from internal combustion engines), NOx and particulates. NOx, which are mixtures of oxides of nitrogen, constitute a major component of these byproducts.
Statistics show that sixty to eighty percent of nitrogen oxides in the atmosphere is produced by mobile sources, whereas the remainder is produced by stationary internal combustion engines. Ever-tightening regulations from governments have kept an unrelenting pressure to develop more efficient engines which produce less pollution. Researchers worldwide have been struggling long and hard to develop such engines. However, this objective has proven difficult to achieve. For example, if engine efficiency is increased by increasing air to fuel ratio so that fuel is completely consumed, as in lean-burn engines, carbon monoxide, hydrocarbon, and particulate emissions are reduced. However, this change is accompanied by a corresponding increase in nitrogen oxide emission. Conversely, if the combustion process is modified to reduce nitrogen oxide production, i.e., by running the engines under rich-bum (fuel excess) conditions, more particulates and hydrocarbons are emitted.
Consequently, researchers have focused their efforts on controlling the emission of harmful materials. These emission control efforts have led to the development of catalytic converters and other catalysts that are very effective in controlling harmful emission. For example, three-way catalysts such as [Pt and/or Pd+Rh]/CeO2xe2x80x94Al2O3 and selective catalytic reduction (SCR) catalysts such as V2O5xe2x80x94TiO2 and Cu-zeolite have been successfully employed to convert NOx to innocuous gases.
In the three-way catalysts, rhodium (Rh) can selectively reduce NOx to N2 in stoichiometric ratio in the absence (or at extremely low concentrations) of oxygen. However, the presence of oxygen rapidly deteriorates the performance of these three-way catalysts. Therefore, this technology is not suitable for controlling NOx emission form lean-burn engines. Furthermore, this technology is ineffective in controlling NOx emission from diesel engines because diesel engines function at lower temperatures, but the available catalysts require high temperatures for optimal performance.
One of the NOx emission control technologies under consideration by diesel and lean-burn engine makers is an NOx storage catalyst with intermittent reduction. With this technology, NOx in exhaust gases is temporarily adsorbed on the catalyst during a lean-burn cycle. The engine is intermittently switched to a rich-bum cycle to reduce the adsorbed NOx to N2 in the absence of oxygen. These catalysts, however, suffer from deactivation by SO2 poisoning from sulfur in diesel fuel.
Another approach being considered for NOx emission control in diesel and lean-burn engines is SCR with NH3/urea. SCR catalysts such as V2O5xe2x80x94TiO2, or [Fe, Cu, etc.]xe2x80x94zeolite, or natural xcex1-Fe2O3 with or without another metal as disclosed in U.S. Pat. No. 4,138,368 issued to Makoto Kiyomiya for example, can use external ammonia to reduce NOx to N2 and H2O. FIG. 1 illustrates a common setup for converting NOx to harmless components using SCR. In this setup, an ammonia tank 20 is connected to a catalytic unit 26 in which NOx are converted to N2 and H2O by reacting with NH3 from tank 20 in the presence of an SCR catalyst. After reacting with ammonia, the exhaust gases can then be safely expelled through the exhaust pipe 24 into the atmosphere. This NOx reduction technique is widely used with conventional electric power plants and other stationary engines. The toxicity and manipulation problems of ammonia, however, has made use of the technology in automobiles or other mobile engines impracticable.
U.S. Pat. No. 5,863,508 issued to Lachman et al. describes a multi-stage catalytic reactor system which allows ammonia to be synthesized onboard a vehicle and then used to reduce NOx to innocuous products. The reactor system includes two units, each of which includes multiple open-ended cells. A portion of the cells in the first unit contains a first stage catalyst, which typically contains 0.01%-5% noble metal (e.g., Pt) on a support (20%-50% Cerium from CeO2 and alumina for the balance). Exhaust gases from combustion are passed through the first unit so that a portion of the NOx in the exhaust gases is reduced to ammonia by the first stage catalyst. The modified gas mixture is then passed to the second unit, wherein the ammonia in the modified gas mixture is reacted with the remaining NOx to yield a converted gas mixture. An external source of ammonia is not needed because the ammonia is generated in the first unit. The passage of the exhaust gases through the first and second units results in conversion of NOx, CO, and hydrocarbons to environmentally benign products.
A similar approach as disclosed in U.S. Pat. No. 6,047,542 issued to Kinugasa et al. and its related patents involves separating engine cylinders into two groups; the first group is regulated to run under rich-burn conditions and the second group under lean-burn conditions. NOx in the exhaust gas from the first group (rich-burn cylinders) is reduced by a three-way catalyst to produce NH3, which is then reacted with NOx in the exhaust gas from the second group (lean-burn cylinders) to produce innocuous products on a NH3 adsorbing-oxidizing catalyst. The need to separate engine cylinders into two groups is due to the fact that the prior art NH3 generation catalysts are not suitable for lean-burn or diesel application.
In one aspect, the invention relates to a catalyst for converting NOx in exhaust gases to NH3 which comprises at least one metal oxide impregnated with at least one noble metal, the metal oxide being selected from a group consisting of Fe2O3, Cr2O3, MgO, La2O3, ZnO, TiOx, and CeO2, and the noble metal being selected from a group consisting of Pt, Pd, Ir, Rh, and Ru.
In another aspect, the invention relates to a catalyst for converting NO, in exhaust gases to NH3 which comprises one or more compounds represented by the formula AB1xe2x88x92xMxO3, wherein A is a rare earth, B is a transition metal, and M is a noble metal.
In another aspect, the invention relates to a catalytic unit comprising at least one catalyst of the present invention.
In yet another aspect, the invention relates to a method of generating NH3 from NOx comprising passing a stream of gas which comprises NOx, oxygen, and at least one reductant through a catalyst of the present invention.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.